Zitat von »Geox19«

How many pumps can a Aquaero have hooked up to it at one time? Does the Aquabus provide power to the pumps or do you still need to hook up the power via molex cable? In what situation would one need to buy a poweradjust 3 or ultra?

You can connect up to 4 mps devices (flow, pressure, d5 pump) to Aquaero via aquabus at the same time. Aquabus doesn't provide power for pump operation, only for controller electronics and you have to hook up the power via molex cable.
Poweradjust is needed if you want to use a pump without PWM or Aquabus with Aquaero and control it via currency.

As Syberon has said the D5 USB is an MPS device and the Aquaero can support a total of four of any of the MPS family, USB flow meter, USB D5,, MPS flow meter etc.
You will just need to first connect each pump to the PC via USB and give it a unique Aquabus address. They all have an address of 12 by default so you could just change one to 13 and another to 14. You then need to set control priority to Aquabus but that may be automatic in the newest software versions. Its best to do this one at a time. Then you can either remove the USB cables or leave them in place if you have enough ports available. Aquabus provides the best control of the pumps though so leaving USB attached doesn't provide any benefit.
Then you can attach the Aquabus cables after first removing all power from the Aquaero including standby USB power from the MB. On powering up it should detect the Aquabus devices and show them all connected with unique addresses in the Aquabus tab.
If you connect multiple MPS devices to Aquabus with the same default address it causes the whole system to go wobbly.

All D5 pumps take their main power from the Molex cable and will not run without it.

Poweradjusts are not really useful for D5 pumps, they were designed for voltage controlled pumps like the DDC 3.2